Have You Been Naughty or Nice: A Privacy-Preserving and Trusted Message To Santa: Partially Blinded Signatures?

I wrote about blinded signatures here:

Do You Remember Carbon Copies? Here’s Blind Signatures.

and where Trent could sign Bob’s message, but where Trent could not see the message he was signing. But what’s to stop Bob from getting Trent to sign something that he doesn’t want to sign? For this, we can use a partially blind signing method, where Trent gets to see the metadata of the message but not the message:

Now, let’s say that Santa has an Elf who digitally signs the messages to Santa. But, we don’t want the Elf to read the letter, but for the Elf to sign that it has been processed by the Elf. But, Santa receives lots of letters, and not just Naughty or Nice ones. So, Bob sends the Elf this Naughty or Nice letter, but adds some metadata to identify that it is a Naughty or Nice letter. The Elf can then read the metadata and know it’s for Santa’s present list, but can see if Bob has been naughty or nice.

So, it could be: